Got an iphong 3g but don't know what to do with it now?

My uncle found an iphone 3g a few months ago he gave it to my little sister and she traded with my big sister for 2 coolpads (bigger phones). Soon my big sister traded the iphone for one of my jackets that she likes. Now my little sister is mad because the two coolpads broke and my mom threw them away she wants the iphone back now. While i was gone she took the iphone and the charger and she put a passcode on iphone, but now the charger is broken and the iphone well i don't know the passcode too it and she will not tell me the passcode unless i let her have the iphone back i'm not going to do that because she is too reckless with phones she does not care about her phone and when it breaks she expects our parents to buy her a new one.it has the sim card and stuff and it's almost dead i don't want to give it back to her because i don't think she deserves it what should i do.

Your uncle 'found' an iPhone and gave it to your sister? Don't you mean your uncle stole an iPhone and palmed it off to your sister, who then traded it for something else?

If you have the phone, just take the sim card out of it and give it to her. The phone ID won't change, and when it is traced back to her, let her deal with it.

Simply restore your iPhone by putting it in DFU mode to make it useable.